(1) Field of the Invention
The present invention relates to a receiver for receiving an amplitude shift keying signal (ASK signal).
(2) Description of the Related Art
As a conventional technology for receiving an ASK signal, there has been proposed a technology, in which a clock extract circuit of data carrier for receiving a signal subjected to ASK modulation, that is, a resonance circuit for receiving an ASK signal is connected to a full-wave rectifying circuit, then its output is discriminated with a threshold (i.e. threshold value) near to a peak value by using a comparator, then the discriminated output is provided to charge and discharge circuit, a time constant of which is selected to be about a carrier frequency, and then the output from the charge and discharge circuit is discriminated by a comparator, thereby extracting a clock (for example, see Japanese Patent Application Laid-Open No. H5-327794).
Further, as another conventional technology for receiving an ASK signal, there has been proposed a technology, in which a received signal is amplified by an amplifier, then the amplified signal is inputted into a comparator having an adjustable threshold, thereby outputting high-frequency rectangular wave from the comparator (for example, see Japanese Patent Application Laid-Open No. 2001-36591).
However, in the former, upon providing the signal to the comparator the threshold is a fixed value, therefore when the level of the signal changes, its way of influencing the threshold changes, thereby causing a problem that the output from the comparator is changed.
In the latter, when the inputted signal is weak (for example, several tens of mV), if the frequency of the signal is high (for example, equal to or higher than 100 kHz), it causes a problem of insufficient gain of an amplifier, thereby not enabling amplification with an amplifier having a simple circuit construction, needing high-performance components or a complicated circuit construction, thereby causing a problem of an increase in electric current consumption.